It is known that increasing the bypass ratio of a conventional turbofan engine can reduce its fuel consumption and consequent level of CO2 emissions. This characteristic has been exploited by engine manufacturers by progressively increasing the bypass ratios of modern turbofan engines.
However, there is a limit to how much the bypass ratio can be increased as eventually the weight and drag penalties associated with the size of the required engine nacelle outweigh the reduction in fuel consumption.
An alternative to the high bypass turbofan engine is the open rotor or unducted fan engine where the rotor or fan is not contained within the nacelle. This enables the fan or propeller to be larger in diameter which increases the bypass ratio while at the same time removing the need for a heavy drag-inducing nacelle.
This in turn allows the open rotor engine to burn significantly less fuel (up to 30% in some instances) and offer associated reductions in emissions when compared to a conventional turbofan engine.
An open rotor engine, although similar to a turboprop engine, is designed to operate efficiently at higher cruise velocities than turboprop engines. The primary difference between turboprop and open rotor engines is that the propeller blades on an open rotor engine have a higher solidity (generally by virtue of the greater number of propeller blades) than those of a turboprop engine. In addition, in contrast to a turboprop engine, an open rotor engine generates a portion of its total thrust from the engine's core exhaust.
A problem with open rotor engines is that they generate higher noise levels than conventional turbofan engines, in which noise from the fan is muffled by the nacelle.
Noise is of particular concern in a preferred open rotor arrangement which comprises two contra-rotating rotor assemblies. The wakes produced by the first (upstream) rotor are ‘chopped’ through by the rear (downstream) rotor. The intensity of the noise emitted by the wake interaction between the front rotor and the rear rotor is proportional to the strength of the wakes produced by the front rotor blades. The strength of the wakes produced by the blades of the front rotor can be reduced by decreasing the loading, or lift, generated by each individual blade. This can be accomplished without compromising the thrust produced by the engine by increasing the number of front rotor blades, i.e. each rotor blade is required to produce less lift in order to produce the same total engine thrust.
In an open rotor engine arrangement the front rotor is subjected to axial Mach numbers equivalent to the forward flight speed of the aircraft. The high inlet velocity combined with the rotational speed of the rotor can result in the air entering the passage between two adjacent rotor blades having an even higher relative Mach number (approximately Mach 0.8). This may result in the flow regime through the open rotor suffering from choking.
In a conventional turbo-fan engine the nacelle and intake to the fan diffuses the flow such that the inlet flow Mach number is lower than the flight speed of the aircraft when in a cruise condition but is higher than the flight speed of the aircraft at take-off or landing. Consequently the problem of choking of fan blades generally does not occur in such engines.
The term choke margin is often used to describe the range of flow conditions relative to the choke point of the rotor. This can be defined as follows:
                              Choke          ⁢                                          ⁢          Margin                =                  (                                                    Choke                ⁢                                                                  ⁢                Flow                            -                              Operating                ⁢                                                                  ⁢                Point                ⁢                                                                                          ⁢                                                                                        ⁢                Flow                                                    Choke              ⁢                                                          ⁢              Flow                                )                                    Eqn        ⁢                                  ⁢                  (          1          )                    
An additional constraint on the rotor blades is that they must have a minimum thickness in order to provide the required structural strength to satisfy the bird strike requirement. This required minimum thickness puts an upper limit on the size of the area (throat area) between two adjacent blades.
The high relative inlet Mach numbers and the limitation in throat area driven by the required minimum blade thickness can result in the airflow through the rotor becoming choked. Choking is particularly prevalent over the inboard portion of the blade since in this region the throat area is smallest and the blade is the thickest. Consequently, in order to avoid the flow becoming choked, it is often necessary to limit the number of blades on the rotor. However, in order to reduce the rotor noise it is desirable to increase the number of blades on the rotor.